Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants are composed of gaseous compounds, such as, for example, the oxides of nitrogen (NOx). Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amount of NOx emitted from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. In order to ensure compliance with the regulation of these compounds, some engine manufacturers have implemented a strategy called Selective Catalytic Reduction (SCR). SCR is a process where a gaseous or liquid reductant (most commonly a urea/water solution) is added to the exhaust gas stream of an engine and absorbed onto a catalyst. The reductant reacts with NOx in the exhaust gas to form H2O and N2, thereby reducing harmful emissions.
Although SCR can be effective, the reductant can be problematic in both cold and hot temperatures. Specifically, urea at a 32% concentration has a freezing point of about −11° C. and, thus, may freeze and become useless in some environments. The freezing temperature may rise with an increase or decrease in concentration, which may occur as water evaporates. Typically, an electric heater is used to thaw the urea. However, after thawing the urea in the immediate vicinity of the heater, the thawed urea may immediately be consumed by the SCR process. If urea is used faster than it can be thawed, an air gap may be created between the heater and the rest of the frozen urea. This air gap may act as an insulator and decrease the effectiveness of the heater in thawing the rest of the urea. Additionally, urea may begin to decompose at a temperature of about 133° C. The decomposition of urea prior to passing through the SCR system may hinder the SCR process. To prevent the decomposition of urea caused by heating, the urea storage tank may be placed away from areas that could exceed the decomposition temperature (i.e. away from the engine).
One method aimed at overcoming the temperature limitations of urea is described in US Patent Application Publication No. 2007/0157602 (the '602 publication) issued to Gschwind on Jul. 12, 2007. Specifically, the '602 publication discloses a tank system having a melting tank and a main tank, the melting tank being at least large enough to hold a cold start volume of reductant. The melting tank is either attached to or disposed within the main tank as a single unit, or is detached and separated from the main tank. The disclosed tank system of the '602 publication passes urea from the main tank to the melting tank where the urea is heated and then passed to a liquid consumer, which discharges the urea into an exhaust gas stream. In the event of a cold start where the urea supply of the main tank is frozen, the melting tank is configured to thaw urea via an electric heating element, giving time for the main tank to melt on its own.
Although the tank system of the '602 publication may suitably thaw the urea within the melting tank, the urea within the main tank may still remain frozen when the melting tank is emptied. Thus, once the urea within the melting tank has been used, the SCR system is once again without a supply of urea. And, the '602 publication fails to address the problem of urea overheating.
The exhaust after-treatment system of the present disclosure solves one or more of the problems set forth above.